Cutting-edge high efficiency microelectronics is vital in a wide range of commercial and military applications. To some extent, it is the driving force behind current semiconductor technology. As we approach sub-10 nm scale in fabrication, revolutionary technology is needed to reach the ultimate miniaturization of high density, high throughput, yet highly energy efficient next generation microelectronics.
In this regard, graphene is the most-researched 2D material because of its unique electronic properties such as a very high carrier mobility. However, graphene lacks an energy band gap, a prerequisite for modern electronics, and is unsuitable for making transistors. Scientists are still trying to design a graphene-based transistor by structure and chemical modifications.
Silicene—the silicon counterpart of graphene—shares many exceptional electronic properties with graphene. The schematics in FIG. 1A (top view) and 1B (side view) illustrate the structure of silicene. Both silicene and graphene possess a Dirac cone and linear electronic dispersion around the F point, and both contain massless Dirac fermions that carry charge. Although both form hexagonal structures, graphene is completely flat, while, as illustrated in the schematics shown in FIGS. 1A and 1B, silicene has a low-buckled honeycomb atomic arrangement of sp2/sp3-like hybridized silicon atoms that is expected to have an easily tunable band gap under an external electric field.
Despite the similarities between graphene and silicene, one of the most notable differences is that silicene has an easily tunable band gap and is fully compatible with current silicon-based microelectronics. In one potentially significant development, a silicene field-effect transistor has recently been demonstrated. See Li Tao et al., “Silicene field-effect transistors operating at room temperature,” Nature Nanotech. 10, 227 (2015). The potential applications of such silicene-based transistors and other silicene-based devices could revolutionize many technological areas such as autonomous systems, communication, sensing, and surveillance.
Currently there is no evidence of existence of natural silicene, nor is there any solid phase of silicon similar to the naturally occurring graphite form of carbon. While the growth mechanism of silicene is still not well understood, molecular beam epitaxy (MBE), one of the most sophisticated film growth methods known at present, is the only technique used to grow silicene.
So far all the experimental characterization of structural and electronic properties of silicene have been performed on such MBE silicene materials. See Lok C. Lew Yan Voon, Silicene, Springer International Publishing Switzerland 2016 M. J. S. Spencer and T. Morishita (eds.), at pp. 3-33. Using MBE, silicene has been deposited on Ag(111), Ir (111), and ZrB2 (111) surfaces at substrate temperature of 200-250° C. See Baojie Feng et al., “Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111),” Nano Letters 12, 3507, (2012); Lei Meng et al., “Buckled Silicene Formation on Ir(111),” Nano Letters 13, 685 (2013); and Antoine Fleurence et al., “Experimental Evidence for Epitaxial Silicene on Diboride Thin Films,” Phys. Rev. Lett. 108, 245501 (2012).
However, MBE formation of silicene requires multiple processing steps in a stringently maintained ultrahigh vacuum environment. Control of nucleation of Si atoms into silicene during MBE can cause deposition of even a single monolayer to take as long as several hours. Consequently, alternative growth methods are needed for potential large scale synthesis and application.
Plasma-enhanced chemical vapor deposition (PECVD) uses reactant gases which are decomposed in a radio-frequency-driven plasma environment. It is a versatile and cost-effective way to deposit various thin film materials in industrial scale.
PECVD has been widely used to deposit amorphous thin films such as amorphous silicon (a-Si), silicon nitride (SiNx), and silicon dioxide (a-SiO2). By introducing hydrogen gas (H2) to dilute silane gas (SiH4), it is possible to deposit nc-Si when H2 to SiH4 ratio exceeds 30. See J. Koh, et al., “Optimization of hydrogenated amorphous silicon p-i-n solar cells with two-step i layers guided by real-time spectroscopic ellipsometry,” Appl. Phys. Lett. 73, 1526 (1998).
The inventors of the present invention have previously used CVD to grow nanocrystalline silicon (nc-Si). See U.S. Pat. No. 9,577,174, to Liu et al., entitled “CVD Nanocrystalline Silicon Thermoelectric Material” (PECVD) and U.S. Pat. No. 9,472,745, to Liu et al., entitled “CVD Nanocrystalline Silicon Thermoelectric Material” (hot-wire CVD, or “HWCVD”). The nc-Si produced in accordance with the inventors' prior inventions has demonstrated record-breaking low thermal conductivity among nonporous silicon material of any type, with a room-temperature thermal conductivity of a factor of three below the minimum thermal conductivity of silicon. When doped by ion implantation, nc-Si also shows promising thermoelectric performance at room temperature.
The inventors of the present invention have used PECVD to deposit nc-Si with ultrafine grain sizes (2-3 nm). Average grain sizes of 3 nm and individual layer thickness of 5 nm have been obtained, as confirmed by both high resolution TEM and X-ray diffraction studies. By controlling RF power, chamber gas pressure, substrate temperature, and H2:SiH4 dilution ratio, the inventors of the present invention have found that grain sizes can be controlled so that a multi-layer nc-Si film, each layer having its own distinct grain size, could be deposited. See '174 patent, supra.